Adaptors with switchable output voltages (such as 5V and 14.75V) are used to recharge different electronic devices, such as a smart phone and a laptop. Typically, a flyback converter is used. The difference of the output voltages is big between a high voltage mode and a low voltage mode, which results in a big difference in a conversion voltage at an auxiliary winding.
FIG. 1 illustrates a conventional supply circuit 100, which uses an auxiliary winding W1 114C and a transistor switch Q1 118 within an auxiliary circuit 104 to form a pre-regulated supply circuit for the main controller 112. Specifically, the conventional supply circuit 100 comprises two sub-circuits including the flyback converter 102 for powering the load 108 and the auxiliary supply circuit 104 for powering the controller 112. The AC power source 110 electrically couples with the flyback converter 102 to provide power to the converter. The flyback converter 102 comprises a transformer T having a primary winding 114A electrically coupled with a switch 116 and a secondary winding 114B coupled to the load 108. The switch 116 is electrically coupled with the main controller 112 such that the main controller 112 is able to control the switching of the switch 116. Further, the auxiliary circuit 104 is electrically coupled with the auxiliary supply circuit 104 via the auxiliary winding 114C of the transformer T 120.
Generally, the voltage on the auxiliary winding 114C is proportional to the output voltage of flyback converter to the load 108. As a result, in a high voltage mode (such as 14.75V), the voltage on the auxiliary winding 114C will be very high, which results in a high voltage difference between the voltage Vcc and the voltage on the switch Q1. This high voltage difference between Vcc and Q1 results in the high losses in the switch Q1 during operation due to the constant sink current from main controller 112, which generally requires a low voltage to be powered properly (such as 5V).